1. Field
This invention relates to the field of chemical engineering, and particularly to the heating of chemical materials. More particularly, the invention pertains to calcination processes requiring heat transfer under temperature controlled conditions.
2. State of the Art
The thermal processing of various ores and other inorganic and organic materials often utilizes a stream of hot flue gas as a source of enthalpy. The flue gas is generally formed by combusting natural gas, coal or oil. The flue gas temperature is controlled to provide the optimum economy and product quality, and may be as high as 2400 degrees F. or more.
In some processes, especially those where indirect heat exchange is used, a high heat transfer rate can be obtained only if the flue gas exits the process at an elevated temperature. A considerable quantity of the enthalpy is thus wasted. At the same time, the resistance to heat transfer limits the throughput of the system. In chemical processes where the reactions must be carried out at temperatures considerably lower than the maximum flue gas temperature, a further limitation is placed upon the flue gas temperature which may be used.
In the calcination conversion of gypsum dihydrate, i.e. CaSO.sub.4 .multidot.2H.sub.2 O, to hemihydrate calcium sulfate, CaSO.sub.4 .multidot.1/2H.sub.2 O, or to anhydrous calcium sulfate, CaSO.sub.4, the hot flue gas may be introduced into the calcination kettle to directly heat the hydrate. In the generally preferred method, however, the heat exchange is conducted indirectly, and hot flue gas does not contact the solids at all. The water of hydration driven off from the solids is discharged from the kettle as a stream separate from the partially cooled flue gases.
Calcination of gypsum to hemihydrate calcium sulfate occurs at temperatures somewhat greater than the normal boiling point of water. However, satisfying the large thermal requirement, i.e. about 500,000 BTU per ton of calcium sulfate hemihydrate produced, necessitates rapid heat transfer to achieve the desired economy. Flue gases at temperatures as high as about 900-2000 degrees F. are used. Higher temperatures may result in deleterious "dead burning", i.e. the conversion of a portion of the gypsum to insoluble calcium sulfate anhydrite. In practice, the temperature and solids residence time in the calcination vessel are controlled to achieve the desired water of hydration without "dead burning" any substantial amount of the calcium sulfate.
Boiling in the calcination vessel typically produces fluidization of the solids. The mixing characteristics in the vessel may approach that of perfect mixing, resulting in nearly uniform temperatures throughout the vessel. The temperature difference across the heat exchange wall will drop as the hot vapor passes from the hot vapor inlet to the vapor outlet. As a result, only a portion of the available enthalpy in the hot vapors are extracted in the calcination vessel. The cooled vapor will be relatively hot, typically about 500-900 degrees F., as it leaves the vessel, and further heat recovery is required before discharge to avoid large heat losses.